This invention relates to a method and apparatus for controlling a continuously variable speed transmission.
In known stepped transmissions, only fixed transmission ratio jumps are possible. However, in order to determine transmission ratio shifts, it is important to know whether the driving takes place uphill or downhill, loaded or empty. Thus, a transmission shift from the third into the fourth gear, which can easily be driven in a level plane, may be much too large on a corresponding incline. This leads to a situation in which the vehicle can accelerate in the third gear to the maximal rotational speed of the driving engine, but can then no longer maintain the achieved speed in the fourth gear. In the case of automatically shifting stepped transmissions, this results in repeated shifting (in the indicated example, between the third and the fourth gear) which can be avoided only by manually selecting a particular gear (in the indicated example, the third gear), using a selector lever.
For continuously variable transmissions, it is known from German Patent Document DE-A1 41 20 540 to simulate a stepped transmission in that fixed transmission ratios are defined. In addition, it is known from German Patent Document DE-A1 42 39 133 to simulate a stepped automatic device by means of a continuously variable transmission also while defining fixed ratios. The above-mentioned problems also occur in this case.
In is an object of the present invention to provide a control arrangement for a continuously variable automatic vehicle transmission which avoids the problem of repeated shifting, without the need for driver intervention by way of the shift selector level.
This object is achieved according to the present invention by including in the control arrangement for continuously variable transmission, a unit for detecting driving situations in which a prospective transmission ratio shift is too large, and a unit for adjusting the preset ratio. The recognition unit activates the adjustment unit as soon as the above-described driving situation (in which repeated, or "pendulum shifting" may occur) is reached. The adjustment unit then changes a preset ratio such that the motive force is adjusted to the present motive resistance; that is, the vehicle can achieve a positive longitudinal acceleration. As a result, advantageously (particularly in the case of an automatic transmission), pendulum shifting is avoided. In the case of manually shifting transmissions, in special driving situations, the transmission ratio is adapted to the existing operating conditions of the vehicle without any resulting loss of the typical characteristics of a stepped transmission.
In a first embodiment of the invention, the recognition unit will recognize if, because of an insufficient amount of motive force, a shift occurs from a preset transmission ratio to a next higher preset transmission ratio (corresponding to a backshift in the case of a stepped transmission). As an indication of an insufficient amount of motive force, the recognition unit preferably determines whether simultaneously a throttle valve position is larger than a limit value and a longitudinal acceleration is less than zero. This type of recognition has the advantage that the intervention takes place only when it is no longer possible to follow the automatic control or the driver's wish.
As soon as an upper engine speed limit is reached, the above embodiment of the adjustment unit changes the transmission ratio so that the rotational transmission input speed is kept constant. As a result, by adjustment of the transmission ratio, the motive force is adapted to the presently existing motive resistance. When the difference between the presently adjusted transmission ratio and the next lower transmission ratio reaches a defined limit value, an upshift is initiated. Thus, despite the adjustment of the transmission ratio, the character of a stepped transmission is maintained.
A second embodiment of the adjustment unit changes the transmission ratio in the described manner until a surplus, or reserve, of motive force exists at the next lower ratio (corresponding to a next higher gear in the case of a stepped transmission).
A third embodiment of the adjustment unit becomes operative before the change to the next higher transmission ratio. From the start, the next higher transmission ratio is determined and adjusted so that a preset margin is maintained between the current rotational speed of the driving engine and its maximum rotational speed. The size of this rotational speed margin is preferably determined as a percentage of the rotational speed jump in the case of the intended change of the transmission ratio and, in this case, may assume values of, for example, between 25% and 50%. It is also an advantage for the rotational speed margin to be determined such that a motive force reserve will always exist. This construction maintains the constancy of a once-adjusted transmission ratio as a typical characteristic of a stepped transmission.
A second embodiment of the recognition unit is based on the determination of a motive force reserve in the next higher gear before this gear is shifted. This determination of the motive force reserve may be made in a known manner, for example, by means of a table, based on the present throttle valve position and the rotational engine speed occurring in the next higher gear. If this value is zero or negative, the transmission ratio may be adjusted correspondingly, even before a shifting operation.
An appropriate adjustment unit is constructed such that a change of ratio is first suppressed. That is, when the driving engine reaches an upper rotational speed limit, the transmission ratio is changed so that the rotational transmission input speed remains constant. This adjustment of the ratio is carried out until a motive force reserve exists at the new lower ratio (corresponding to the next higher gear in the stepped transmission). This embodiment reduces the transmission ratio adjustment to the minimum necessary, and the character of a stepped transmission is also maintained.
A second embodiment of the adjustment unit will now not affect the present gear; rather, the transmission ratio adjustment at the next lower ratio is altered before a change thereto to such an extent that a motive force reserve exists there. In this case also, a transmission ratio adjustment takes place only to the extent absolutely necessary, and the character of a stepped transmission is maintained because no adjustment is carried out on the currently selected transmission ratio.
The described recognition units are advantageously constructed as part of a microprocessor-driven control unit. Of course, they may also be implemented by discrete comparators, such as differential amplifiers, in which case a first comparator is provided for the throttle valve position; a second comparator is provided for longitudinal acceleration; and a third comparator is provided for rotational engine speed. In this case, the first comparator compares the throttle valve position with a lower limit value; the second comparator compares the longitudinal acceleration with the maximum value zero; and the third comparator compares the rotational engine speed with a maximum value. The comparators each emit a signal when the comparison discloses identify of input values.
The above-described effects and advantages apply to a manually shifted transmission as well as to an automatically shifted simulation of a stepped transmission by means of a continuously variable transmission. Analogously, they also apply to the process according to the invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.